1. Field of the Invention
The present invention relates to a color thermal printer in which heat applied by a thermal print-head is used to heat ink donor film to transfer ink to a print receiver of paper or other such material. It particularly relates to a drum type color thermal printer in which the print receiver is moved by the rotation of a drum.
2. Description of the Prior Art
In a color thermal printer, a print receiver such as paper, for example, and an ink donor film are fed between a thermal print-head and a platen roller, and the thermal print-head is heated to transfer the ink to the receiver.
FIGS. 5 and 6 show an example of the print receiver transport mechanism and printing section in a conventional color thermal printer.
FIG. 5 shows an example of a color thermal printer which employs a reciprocating receiver transport system. In the drawing, a paper receiver 10 is pinched between two sets of pinch rollers 12 and 14, and is moved backwards and forwards by the rotation of these pinch rollers. The rotation of the pinch rollers 12 and 14 feeds the print receiver 10 through a gap between a platen roller 16 and the thermal print-head 18 which constitutes a transfer section.
An ink donor film 20 is also fed through this transfer section, on the print receiver 10. This ink donor film 20 is moved in unison with the movement of the print receiver 10. The heat of the thermal print-head 18 is used to selectively transport ink from the ink donor film 20 to the print receiver 10 located between the platen roller 16 and the thermal print-head 18.
FIG. 6 illustrates a color thermal printer in which the print receiver 10 is moved in one direction only by a rotational mechanism. Here, the platen roller is constituted by a platen drum 28. The platen drum 28 is provided with a gripper 30, which clamps the print receiver 10 onto the surface of the platen drum 28. Film rollers 22 and 24 are provided for the same purpose described with reference to FIG. 5, i.e. to transport the thermal print-head 18 and the ink donor film 20.
The operation of the printers shown in FIGS. 5 and 6 will now be described. With reference first to FIG. 5, the print receiver 10 is drawn out by the pinch rollers 12 and fed, in contact with the surface of the platen roller 16, through the transfer section between the thermal print-head 18 and the platen roller 16. The transport of the print receiver 10 is continued to bring the leading edge of the print receiver between the second set of pinch rollers 14. During this part of the operation the thermal print-head 18 is retracted away from the platen roller 16, and the ink donor film 20 is moved on until it is in the prescribed position.
Next, the thermal print-head 18 is held against the platen roller 16 at a prescribed pressure and the thermal transport process begins. During the thermal transport process the print receiver 10, held by the frictional force of the pinch rollers 12, is printed while it is being transported in the direction indicated by the arrow 100 by the rotation of the pinch rollers 14 and the platen roller 16. The pinch rollers 12 and 14 and the platen roller 16 are rotated in unison to keep the print receiver 10 smooth and taut, and the ink donor film 20 is wound onto the film roller 22 at a rate which matches the speed at which the print receiver 10 is being transported. At the point at which the transfer of a color is completed, it is necessary that the trailing edge of the print receiver 10 is still being held between the pinch rollers 12. To print the next color, the receiver 10 has to be transported back to a predetermined transfer start position. To do this, the thermal print-head 18 is retracted away from the platen roller 16 and the ink donor film 20 is moved to a prescribed position (the position at which the ink of the desired color is located). During this operation the print receiver 10 is transported in the direction indicated by the arrow 200 by the reverse rotation of the pinch rollers 12 and 14 until the home position is reached. Each color is thus printed, i.e. thermally transferred, by the above cycle of operations, so to completion of the thermal color printing is effected by repeating this cycle of operations a number of times which corresponds to the number of colors which have to be transferred. A three-color printing (yellow, magenta and cyan, for example) would be accomplished by repeating the above cycle of operations three times.
Next, with reference to the operation of the conventional apparatus illustrated by FIG. 6, with the print receiver 10 held in position on the platen drum 28 by the gripper 30 the platen drum 28 is rotated until the home position is reached. During this rotation the thermal print-head 18 is maintained in a position of retraction from the platen drum 28 and the ink donor film 20 is moved until it is at the necessary position for commencing the transfer printing. The rotation of the platen drum 28 is stopped when it reaches the home position, and the thermal print-head 18 is brought into contact with, and presses against, the platen drum 28 at a prescribed pressure, and the ink transfer begins. When the transfer of a color is completed, which is when the platen drum 28 has described one revolution in the direction indicated by the arrow 300, the next color is transferred by another revolution of the platen drum 28 in the direction indicated by the arrow 300, starting from the home position. Thus, a three-color transfer printing operation would involve three such revolutions by the platen drum.
The conventional color thermal printers described above have the following drawbacks. In the case of the reciprocating system illustrated in FIG. 5, the print receiver 10 is transported by the frictional force of each of the two sets of pinch rollers 12 and 14. To ensure that the print receiver 10 is maintained in a state of tension during its transportation requires that the pressure and frictional force of each of the sets of pinch rollers 12 and 14 be maintained at a constant level, for which an adjusting mechanism is needed. Also, to prevent deviation in color registration starting from the transfer of the second color the print receiver 10 has to be positioned precisely, but with these conventional color thermal printers positional detection and alignment of the print receiver 10 is difficult. Since the unidirectional transport system used in the color thermal printer shown in FIG. 6 is arranged so that the transfer of one color is effected with one revolution of the platen drum 28, it solves the positional alignment problem of the apparatus of FIG. 5. However, the diameter of the platen drum 28 has to be determined in accordance with the length of the print receiver 10 sheets in the direction of transportation. As such, an increase in the size of the print receiver 10 has to be accompanied by a corresponding increase in the diameter of the drum 28, and hence in the radius of curvature r of the drum. The result is a marked increase in the area of contact between the thermal print-head 18 and the platen drum 28, which in turn results in parts of the thermal print-head 18 other than the printing portions coming into contact with the platen drum 28. The consequence of this is that when used as a line printer, the thermal print-head 18 has to be pressed against the platen drum 28 with a very large force in order to produce good-quality printing, which makes the thermal print-head susceptible to damage.
Moreover, an increase in the size of the platen drum 28 has to be matched by a corresponding increase in the size of the thermal print-head 18, thereby increasing both the cost and the overall size of the apparatus.